RADIATION FROM ACCELERATED ALFVEN SOLITONS IN INHOMOGENEOUS PLASMAS

In a weakly inhomogeneous plasma, the large-amplitude Alfvén waves propagating parallel to the ambient magnetic field are shown to evolve into accelerated Alfvén solitons. Nonlinear interaction of the accelerated Alfvén solitons with the Langmuir waves results in the emission of coherent radiations at the frequency ω0 ≃ (ωA + ωL), where ωA and ωL are the Alfvén wave and the Langmuir wave frequencies, respectively. Analytical expression for the power radiated per unit solid angle from a soliton is derived for two inhomogeneity profiles, namely the linear profile and the parabolic profile. For the case of uniform plasmas, the emission occurs via a decay-type process or resonant modes. In the presence of inhomogeneity, nonresonant modes provide a new channel for the emission of radiation. The power radiated per unit solid angle is computed for the parameters relevant to comet Halley's plasma environment; for the nonresonant modes it is found to be several orders of magnitude higher than that for the case of resonant modes. Applications of our model to other astrophysical systems are pointed out.